Every employee should have a specific, written job description. Simply negotiating one does more to dispel a sense of powerlessness than anything else. It is a contract that you help write. You can make objections and insist upon what you do want. If there is a compromise, it's because you agreed to it. With a clear job description, your expectations are spelled out, as are your boss's.
A good job description is time limited. Set a specific date for a review and revision based on your mutual experience with this initial job description. If you and your boss can't agree on what your job description should be, look for another job, either within the same company or outside. Even in these tough economic times, it is important that your job be a source of satisfaction and respect.